I know it's a little silly to reply to my own post, but since it got bumped, I wanted to add something....
I used a poem someone posted on GC for my little (thanks, everyone!), but during Senior Farewell Week, I needed a poem for my big sis. She and I are really, really close, so it needed to be special. I was bored while studying in the library one night, and started mousing around in the library.... To make a long story short, I found a great book called "Big Sister and Little Sister" by Charlotte Zolotow. During Senior Week, all the littles read their poems (or in my case, stories) to their senior bigs in front of the whole house, and when I read that story, not only was I crying so hard I could hardly finish it, but when I finished, we had to take a break to pass the tissues around the whole room!
Anyway, if anyone is looking for something for their big sis, this story is a beautiful thing to give to them.